If the bulge is not all the way around the case, there are some guns that are unsupported where the feed ramp meets the chamber. With that, you will typically see a small bulge, shaped like an open-mouth, kind of round on the semi-circular top (looking at the side of the casing while it is standing on its base).
The flattened and cratered primer does look like overpressure, and it may be that the load was so hot, there was still pressure as it was ejecting, causing the bulge. It may have been a load someone was working up. I hope whoever it is realizes that anything like hotter ammo on a hot day can cause the pressures to spike, thus possibly blowing the base off the case before it is ejected!
One other possibilty could be water that might have frozen thus expanding the case. However, that would have nothing to do with the flattened, cratered primer!